1. Field of the Invention
The present invention relates generally to a rotary drilling arrangement for mitigating pressure-differential sticking of a drill string in a wellbore. More particularly, the subject invention concerns a method and apparatus for drilling deviated wellbores, such as in extended reach drilling, which are particularly designed to reduce the chance of pressure-differential sticking of the drill string by grinding and reducing the size of the cuttings generated by the drilling operation.
Extended Reach Drilling is concerned with rotary drilling procedures to drill, log and complete wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than currently being achieved by conventional directional drilling practices. The success of extended reach drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Extended reach drilling offers significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) tapping sections of reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle extended reach directional drilling. In greater particularity, hole inclinations of 60.degree. or greater, combined with long sections of hole or complex wellbore profiles present significant problems which need to be overcome in extended reach drilling. The force of gravity, coefficients of friction, and mud particle settling are the major physical phenomena of concern.
As inclination increases, the available weight from gravity to move the pipe or wireline string down the hole decreases as the cosine of the inclination angle, and the weight lying against the low side of the hole increases as the sine of the inclination angle. The force resisting the movement of the drill string is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the hole at inclination angles up to approximately 60.degree.. At higher inclination angles, the drill strings will not lower from the force of gravity alone, and must be mechanically pushed or pulled, or alternatively the coefficient of friction can be reduced. Since logging wirelines cannot be pushed, conventional wireline logging is one of the first functions to encounter difficulties in this type of operation.
Hole cleaning also becomes a more significant problem in high angle bore holes because particles need fall only a few inches to be out of the mud flow stream and to come to rest on the low side of the hole, usually in a flow-shaded area alongside the pipe. This problem is also encountered in substantially vertical wellbores but the problem is much worse in deviated wellbores. In deviated wellbores the drill string tends to lie on the lower side of the wellbore and drill cuttings tend to settle and accumulate along the lower side of the wellbore about the drill string. This condition of having drill cuttings lying along the lower side of the wellbore about the drill string along with the usual filter cake on the wellbore wall presents conditions susceptible for differential sticking of the drill pipe when a porous formation is penetrated that has internal pressures less than the pressures existing in the borehole.
Cuttings generated by rock bits are usually less than 1/2" in size and are usually plate-like in structure. A second source of cuttings, which are not really cuttings from the bit, are those generated by sloughing or by erosion of the borehole wall, and these are frequently 1" to 11/2" in length and thicker than a drilled cutting. In general, the larger the cutting size, the more difficult it is to transport it in the mud stream. In mitigation of this, it should be pointed out that some regrinding of the cuttings normally takes place in all rotary-drilled holes by the drill string, particularly the drill collars, by crushing between the rotating pipe and the wall of the hole.
This settling of cuttings is particularly significant in the near horizontal holes expected to be drilled in extended reach drilling. Present drill strings of drill pipe body, tool joints and drill collars are usually round and rotate concentrically about a common axis. If the pipe rotates concentrically around the same axis as the tool joints which are normally positioned against the solid wall and act as bearings for the rotating string, then a long "keyseat" is developed as the pipe is buried and beds itself into the cuttings and wall cake. A similar action of a drill string rotating about a concentric axis in a thick wall cake in a vertical hole could produce the same results. If differential pressure (borehole mud pressure less formation pore pressure) exits opposite a permeable zone in the formation, then conditions are set for the pipe to become differentially wall stuck. In both cases, the pipe is partially buried and bedded into a mass of solids, and can be hydraulically sealed to such an extent that there is a substantial pressure difference in the interface of the pipe and the wall and the space in the open borehole. This hydraulic seal provides an area on the pipe for the pressure differential to force the pipe hard against the wall. The frictional resistance to movement of the pipe against the wall causes the pipe to become immovable, and the pipe is in a state which is commonly referred to as differentially stuck.
2. Discussion of the Prior Art
Pressure-differential sticking of a drill pipe is also discussed in a paper entitled "Pressure-Differential Sticking of Drill Pipe and How It Can Be Avoided or Relieved" by W. E. Helmick and A. J. Longley, presented at the Spring Meeting of the Pacific Coast district, Division of Production, Los Angeles, Calif. in May 1957. This paper states that the theory of pressure-differential sticking was first suggested when it was noted that spotting of oil would free pipe that had stuck while remaining motionless opposite a permeable bed. This was particularly noticeable in a field wherein a depleted zone at 4300 feet with a pressure gradient of 0.035 psi per foot was penetrated by directional holes with mud having hydrostatic gradients of 0.52 psi per foot. In view thereof, it was concluded that the drill collars lay against the filter cake on the low side of the hole, and that the pressure differential acted against the area of the pipe in contact with the isolated cake with sufficient force that a direct pull could not effect release. This paper notes that methods of effecting the release of such a pipe include the use of spotting oil to wet the pipe, thereby relieving the differential pressure, or the step of washing with water to lower the pressure differential by reducing the hydrostatic head. Field application of the principles found in a study discussed in this paper demonstrate that the best manner for dealing with differential sticking is to prevent it by the use of drill collar stabilizers or, more importantly, by intentionally shortening the intervals of time when pipe is at rest opposite permeable formations.
Barrington, U.S. Pat. No. 4,060,140 discloses an arrangement for preventing a buildup of cuttings or debris in underwater oil wells. During drilling, cuttings from the bottom of the well are carried therefrom in a drilling mud solution which is pumped downwardly through the tubular drill string and circulated upwardly in the annulus between the drill string and the borehole, wellhead assembly and riser pipe string to the water surface. These cuttings and other debris from the bottom of the well can be delivered to the water surface provided the proper fluid velocity, mud weight and annulus areas are compatible.
However, when the annulus area between the outer diameter of the drill string and the inner diameter of the riser pipe string is very large in comparison to the annulus between the exterior of the drill string and the wall of the borehole and inner surfaces of the wellhead assembly, the drilling mud can lose the desired velocity or flow rate in the annulus between the drill string and the riser pipe necessary to convey the cuttings and debris upwardly through the riser pipe string to the water surface for removal from the drilling mud.
Accordingly, to alleviate this problem, Barrington purposes a drilling tool in the form of a primary tubular member having an internal diameter a predetermined amount greater than the external diameter of a drill string which is run downwardly through a conventional riser pipe string and underwater wellhead assembly. The drilling tool includes an external shoulder formed on the lower end portion thereof for engagement with a corresponding internal shoulder at the juncture of the riser pipe string and the wellhead assembly for supporting the tool independently of the drill string. The tool includes a second tubular member connected at its lower end to the lower end of the primary tubular member. An annular space, open at its upper end, is thus provided between the two tubular members of the tool for trapping cuttings and debris which may settle out from circulating or non-circulating mud in the annulus between the drill string and the riser pipe string above the tool. The tool is adapted to be retrieved with the drill string upwardly through the riser pipe string to the water surface where any cuttings and debris trapped therein can be disposed of without falling back to the bottom of the borehole. Accordingly, although this patent is concerned with a problem similar to that addressed by the present invention, the proposed solution is quite different from that disclosed and taught herein.